1. Field of the Invention
This invention relates generally to Michelson interferometers. More particularly, it relates to apparatus and techniques for retrieving WDM telemetry signals and other device testing where a Mach-Zehnder interferometer is conventionally used.
2. Background of Related Art
Mach-Zehnder interferometers are known. In one conventional Mach-Zehnder device, the ends of two fibers are fused together and connected to a detector to measure output light intensity. The output light intensity is dependent on phase displacement.
Early Mach-Zehnder devices were limited in accuracy due to the inability to utilize polarized light. However, in later fiber optic Mach-Zehnder interferometers such as the commercially available Model HP 11980 from Hewlett Packard, and/or that disclosed in U.S. Pat. No. 4,759,627, polarization controllers are utilized to control the polarization of the light transmitted through the legs of the Mach-Zehnder interferometer. A phase difference between the output light waves causes a change in the output light intensity, which indicates a measure of the interference between the optical signals propagating in the two arms of the interferometer.
Fiber optic Michelson interferometers are also known. In a conventional Michelson interferometer, light is transmitted through two optical parts (e.g., single mode optical fibers), and the phase angles for the light waves in the two unequal lengths are compared. Like in Mach-Zehnder interferometers, a difference in the phase angle is used as a representation of a measurement of the quality of light propagating in the two paths.
A conventional Michelson interferometer is shown in FIG. 6. (See also FIG. 1(a) of J. A. Armstrong, Theory of Interferometric Analysis of Laser Phase Noise, J. Optical Soc. of Am., Vol. 56, No. 8 (1966), the entirety of which is expressly incorporated herein by reference.
In particular, in FIG. 6, a fiber optic Michelson interferometer 600 is formed utilizing a 3 dB coupler 605 to split an incoming optical signal from an incoming length of single mode fiber optic cable 607 into two separate paths. A first path is formed by a first length of single mode fiber optic cable 613, and a second path is formed by a second length of single mode fiber optic cable 615. The lengths of fiber optic cable in the first path 613 and in the second path 615 are similar, but preferably the length (or other property) of the fiber optic cable in the second path 615 is influenced for measurement.
In operation, light traveling into the input path 607 is evenly split between the first path 613 and the second path 615. The light in each of the first and second paths 613, 615 travels down the single mode optical fiber until impinging upon respective reflectors 609, 611. The reflectors 609, 611 reflect 100% of the light back into the single mode fiber, effectively in the same polarization as the light was in the outgoing direction.
It is known that there is polarization mode dispersion in single mode fiber optics, e.g., as discussed in B. W. Hakki, Polarization Mode Dispersion In A Single Mode Fiber, J. of Lightwave Tech., Vol. 14, No. 10 (Oct. 1996), the entirety of which is explicitly incorporated herein by reference. As a result, the lengths of the fiber optic in the first and second paths 613, 615 are typically relatively short to minimize any change in the polarization due to polarization mode dispersion of the fiber optics. However, to ensure polarization to improve accuracy, polarization controllers 647, 637 are typically used in the first and second paths 613, 615, respectively.
The reflected (and polarization controlled) light from the second path 615 is coupled with the reflected (and polarization controlled) light from the first path 613 by the coupler 605, and combined into one single mode fiber optic path 617. A suitable detector 603 for detecting the amplitude of the transmitted light is placed at the end of the output path 617. In accordance with known principles, a measurement output by the detector 603 is proportional to a quantity of the light that propagates in the two paths.
The conventional fiber optic interferometers such as the known configurations of Mach-Zehnder interferometers are expensive and complicated, due largely to the need for a polarization controller.
There is a need for a less complicated and less costly configuration for a fiber optic interferometer.
In accordance with the principles of the present invention, an uncorrelated Michelson interferometer comprises an optical coupler which splits an incoming light signal into two paths. A first fiber optic path from the optical coupler is formed by a single mode optical fiber having a first length. A second fiber optic path from the optical coupler is formed by a single mode optical fiber having a second length significantly longer than the first length. The optical coupler combines reflected light from the first fiber optic path and reflected light from the second fiber optic path to cause interference therebetween.
A method of causing uncorrelated interference comprises splitting an input light signal between a first single mode optical fiber path and a second single mode optical fiber path. The second single mode optical fiber path has a significantly longer optical length than the first single mode optical fiber path. Light is reflected in an opposite direction at an end of the first single-mode optical fiber path. Light is reflected in an opposite direction at an end of the second single mode optical fiber path. The reflected light is combined to cause uncorrelated interference therebetween.